This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Application No. 2001 1401/01 filed in Switzerland on Jul. 27, 2001; the entire content of which is hereby incorporated by refernce.
The present invention relates to the technical field of generators for producing electrical power. It relates in particular to a protective and monitoring apparatus for a generator as claimed in the precharacterizing clause of claim 1, and to the use of such a protective and monitoring apparatus.
A protective and monitoring apparatus of said type is known, of example, from the document EP-A1-0 271 678.
As described in the document EP-A1-0 271 678 mentioned initially and in the subsequent document DE-A1-197 42 622, shaft voltages and shaft currents can occur for various reasons on the generator shaft of a generator, which is normally part of a turbine set, whose frequency spectrum extends from direct current up to frequencies which are related to the rotation frequency of the generator, its method of construction (number of pole pairs) and to the frequencies of the generator excitation system.
The shaft voltages and shaft currents represent a danger to various components of the generator and can lead to damage to the generator if they are not reduced to a tolerable extent or no protective measures are taken. To this end, special precautions have been taken in the past on the generator shaft by, for example, installing isolating gaps at the non drive end of the generator and by connecting the generator shaft via brushes or copper blades to ground potential at the drive end. In order to ground high-frequency voltages in a controlled manner, it has also been proposed for the non drive end of the generator shaft to be AC-coupled via a capacitance to the ground potential, by means of a sliding contact.
The shaft voltages and shaft currents may, however, also be used to monitor the functional reliability and serviceability of the shaft run and of the shaft grounding. The document EP-A1-0 271 678 mentioned initially in this context describes a monitoring and evaluation circuit which creates a current path by connecting a resistance in parallel with a discharge capacitance at the non drive end of the generator by evaluating frequency components which are typical of the machine in the currents flying through this path. The RC combination provides a reliable connection between the shaft and ground potential, reduces not only static charges but also low-frequency currents and relatively high-frequency voltages to levels which, for example, are not dangerous for the shaft bearings. The introduction of parallel resistance could even mean that there is no need for resistive grounding at the drive end.
Another proposal (DE-A1-0 197 42 622) comprises measuring the shaft current, which flows through the grounding path of the generator shaft, at the drive end and of determining the waveform or frequency of the shaft current, from which the cause of the shaft current can then be derived.
Both high currents and high voltages can occur in the grounding path, and must be coped with. EP-A1-0 271 678 proposes a series-connected fuse in order to switch excessively high currents. However, no precautions are taken against excessively high voltages, such as those which occur when the generator shaft comes into contact with a low-impedance high-voltage source.
DE-A1-197 42 622 does not take any precautions either against excessively high currents or against excessively high voltages.
One object of the invention is thus to provide a protective and monitoring apparatus for a generator shaft, which overcomes the disadvantages of the previous solutions and, while at the same time having a simple design and versatile applicability, is protected against overvoltages on the generator shaft, as well as specifying an advantageous use for the protective and monitoring apparatus.
The object is achieved by the totality of the features of claims 1 and 9. The essence of the invention is to provide at least one means for bridging the RC parallel circuit, which deliberately short-circuit the RC parallel circuit when overvoltage occurs on the generator shaft, thus reducing the voltage dropped across the apparatus to a tolerable level.
According to a first preferred refinement of the invention, a measurement resistance is connected in series with the parallel circuit, between the parallel circuit and the grounding connection, and in that the series of circuits formed by the measurement resistance and the parallel circuit can be bridged by the bridging means. This makes it possible to measure the shaft current flowing away to ground in a simple manner, and allows not only its time profile but also the frequencies which occur to be evaluated and to be used for monitoring the shaft grounding.
In order to achieve protection against excessive currents, it is advantageous if, according to a second refinement of the invention, a fuse is connected in series with the parallel circuit, between the parallel circuit and the contact apparatus, and if the series circuit formed from the measurement resistance, the parallel circuit and the fuse can be bridged by the bridging means. In this case, it has been proven to be advantageous for the fuse to have rated value of approximately 2A.
In order that static charges can still be dissipated from the generator shaft to ground even after the fuse has blown, it is expedient, according to a further refinement of the invention, for a high-value resistance to be connected in parallel with the fuse.
The overvoltage protection is particularly safe and requires little maintenance if the bridging means comprise a controllable switching element, in particular a triac, which is preferably switched on when the voltage which is dropped across the protective monitoring device exceeds approximately 32 V.
One preferred use is characterized in that the voltage which is dropped across the protective and monitoring apparatus is limited by switching on the bridging means when said voltage exceeds a predetermined value.
In particular, the voltage which is dropped across the protective monitoring apparatus and/or the current which is flowing through the protective and monitoring apparatus is measured, and its time profile and the frequency distribution which occurs are evaluated.